Critical Nanoaggregate Concentration of Asphaltenes by Direct-Current (DC) Electrical Conductivity €
By Zeng, Huang; Song, Yi-Qiao; Johnson, David L. & Mullins, Oliver C.
Published in Energy & Fuels
2009
Abstract
The critical nanoaggregate concentration (CNAC) of asphaltenes in toluene has been studied by a variety of methods recently. Here, we explore low-frequency electrical impedance measurements to detect and quantify nanoaggregate formation of asphaltenes. The Nyquist and Bode plots confirm the frequency range necessary for the dominance of (organic) ion conduction as opposed to reactive impedance. Impedance measurements are made as a function of the asphaltene concentration in toluene. We perform ionic conduction measurements at low frequency to avoid electrode polarization effects and then extrapolate to obtain direct-current (DC) conductivity. In a plot of DC conductivity versus asphaltene concentration, we see a clear break between two linear regions that is attributed to nanoaggregate formation. Very close agreement with high-Q ultrasonic measurements is shown for two petroleum asphaltenes with different CNACs. In addition, this work is shown to be consistent with previous alternating-current (AC) conductivity measurements. Measurements on aqueous salt solutions are used to validate measurements of the mole fraction of asphaltenes ionized in toluene, which is ∼10-5. The plausible identity of these ions is discussed. A comparison of conductivity at concentrations below and above CNAC indicates that the aggregation number is small (<10) in agreement with previous findings. Resin shows no aggregation and is also much less conductive than asphaltenes. We also observe a break in the slope at higher asphaltene concentrations, where nanoaggregate clustering has been observed.